Heterogeneous Structure in Diesel Fuel Sprays

Abstract

A laser 2-focus velocimeter (L2F) has been applied for measurements of velocity and size of droplets in diesel sprays. The maximum data acquisition rate of 15 MHz has been achieved by using FPGA in order to capture every droplet which appears in the micro-scale measurement volume. A method of evaluating the mass flow rate of droplets was proposed, and the distance between droplets was adopted as an indicator of the number density of droplets and the heterogeneous structure of sprays. The diesel fuel spray was injected intermittently into the atmosphere by using a 5-hole injector nozzle. The orifice diameter of the injector nozzle was 0.113 mm, and the injection pressure was set at 40MPa by using a common rail system. Measurements were conducted in the spray on 10 planes from 5 to 25 mm downstream from the nozzle exit. It was shown that the velocity of droplet was the highest at the spray center and decreased towards the spray periphery. The size of droplet at the spray center decreased downstream and that at spray periphery increased downstream. The mass flow rate near the spray center was larger than that in the spray periphery region. It was confirmed that the fuel mass per injection evaluated by the proposed method based on the L2F measurement was nearly equal to the injected mass in a plane further than 15 mm from the nozzle exit. The probability density of droplet with a distance shorter than 5 m to the adjacent droplet increased remarkably near the spray center 5 and 12 mm downstream from the nozzle exit. It is thought that a region of high number density of droplet exists at the spray center near the spray tip when the spray penetration is shorter than 15 mm.